This is fairly obvious but I did not think about it until it was too late. I traveled recently and my newly purchased American Recorder CO2 cartridges were taken away from me. As I said, thats not surprising. I was also told that even if it was packed in a checked bag, it would not be allowed. Not sure if that is true or not.

That prohibition has been in place for at least the last 20 years- nothing to do with the current security climate. For many years I was on a project requiring compressed CO2 gas cartridges, and I had to do a lot of flying. It was extremely frustrating to arrange for local sources in tiny towns, but there you go. Interestingly, I could ship the stuff as air freight, often travelling on the same plane I was. Don't look for logic or common sense in all the regs you live by.